Use low entropy for studies that send experiment IDs to Google properties.

base/metrics/field_trial.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // FieldTrial is a class for handling details of statistical experiments
 // performed by actual users in the field (i.e., in a shipped or beta product).
 // All code is called exclusively on the UI thread currently.
 //
 // The simplest example is an experiment to see whether one of two options
 // produces "better" results across our user population.  In that scenario, UMA
@@ skipping 375 matching lines @@
       const int year,
       const int month,
       const int day_of_month,
       FieldTrial::RandomizationType randomization_type,
       int* default_group_number);
 
   // Same as FactoryGetFieldTrial(), but allows specifying a custom seed to be
   // used on one-time randomized field trials (instead of a hash of the trial
   // name, which is used otherwise or if |randomization_seed| has value 0). The
   // |randomization_seed| value (other than 0) should never be the same for two
-  // trials, else this would result in correlated group assignments.
-  // Note: Using a custom randomization seed is only supported by the
-  // PermutedEntropyProvider (which is used when UMA is not enabled).
+  // trials, else this would result in correlated group assignments.  Note:
+  // Using a custom randomization seed is only supported by the
+  // PermutedEntropyProvider (which is used when UMA is not enabled). If
+  // |override_entropy_provider| is not null, then it will be used for
+  // randomization instead of the provider given when the FieldTrialList was
+  // instanciated.
   static FieldTrial* FactoryGetFieldTrialWithRandomizationSeed(
       const std::string& trial_name,
       FieldTrial::Probability total_probability,
       const std::string& default_group_name,
       const int year,
       const int month,
       const int day_of_month,
       FieldTrial::RandomizationType randomization_type,
       uint32_t randomization_seed,
-      int* default_group_number);
+      int* default_group_number,
+      const FieldTrial::EntropyProvider* override_entropy_provider);
 
   // The Find() method can be used to test to see if a named trial was already
   // registered, or to retrieve a pointer to it from the global map.
   static FieldTrial* Find(const std::string& trial_name);
 
   // Returns the group number chosen for the named trial, or
   // FieldTrial::kNotFinalized if the trial does not exist.
   static int FindValue(const std::string& trial_name);
 
   // Returns the group name chosen for the named trial, or the empty string if
@@ skipping 113 matching lines @@
 
   // List of observers to be notified when a group is selected for a FieldTrial.
   scoped_refptr<ObserverListThreadSafe<Observer> > observer_list_;
 
   DISALLOW_COPY_AND_ASSIGN(FieldTrialList);
 };
 
 }  // namespace base
 
 #endif  // BASE_METRICS_FIELD_TRIAL_H_